Metabolic responses to zinc stress in Amaranthus caudatus roots and leaves

During the last decades, pollution with heavy metals (HM) became an important stress factor for plants. However, due to their biochemical plasticity, plants can adjust their metabolism to ensure survival under changing environmental conditions. In the most comprehensive and straightforward way these metabolic shifts can be addressed by the untargeted mass spectrometry-based metabolomics approach. However, although this methodology is well-established for cadmium-related responses, comprehensive metabolomics studies of Zn-induced metabolic shifts are still missing. Therefore, here we propose, to the best of our knowledge, the first gas chromatography-mass spectrometry (GC-MS)-based metabolomics study of Zn2+-induced stress responses in amaranth. Thereby, 419 metabolites could be annotated and 144 of them were unambiguously identified. The metabolic shifts were organ-specific and more pronounced in roots. The most of the responsive metabolites were up-regulated and dominated with sugars and, to a lesser extent, TCA-related organic acids (a total of 51 and 41, respectively) that could be attributed to their involvement in osmoregulation, ROS scavenging and complexation of Zn2+ ions. Galactose was the most Zn2+-responsive root sugar that indicated its possible role in Zn2+ ion binding to the root cell walls. A 59-fold increase of gluconate content in roots clearly indicated its involvement in Zn2+ chelation. A high Zn2+–induced up-regulation of salicylic acid in roots and shoots suggested a key role of this hormone in the activation of Zn2+ stress tolerance mechanisms. Thus, our study provides the first insight in the general trends in Zn-induced biochemical rearrangements and main adaptive metabolic shifts in A. caudatus plants.